Isolated communication technology that uses magnetic couplers has received attention because of its advantages such as higher speed, lower power consumption, and smaller circuit scale as compared to photo couplers. In the insulation communication technology, it is a common technique to detect a rising edge and a falling edge of an input signal and to transfer the edge detection signal from the primary side to the secondary side of a transformer coil.
With earlier techniques, detection signals of a rising edge and a failing edge have been transferred using different transformer coils, resulting in the difficulty of increased circuit scale because of the necessity to provide two sets of transformer coils. This difficulty can be solved by generating an edge detection signal having different waveforms for the rising and falling edges of an input signal and transferring the generated edge detection signal to a secondary side. Thus, the edge detection signal can be transferred using a set of transformer coils.
In methods that have been proposed so far, for example, two short consecutive pulses are generated by a detection of a rising edge, and one short pulse is generated by a detection of a falling edge, or, a long pulse is generated by a detection of a rising edge, and a short pulse is generated by a detection of a falling edge. With these methods, however, a certain time is required for a logic circuit provided on the secondary side to distinguish a rising edge from a falling edge, resulting in an increase of the delay time from an edge change in the input signal until an edge change in the output signal. Also, when the duty ratio of the input signal is increased or decreased so that short-width pulses (hereinafter, “short pulses”) are generated, errors may occur.
A method developed in recent years has enabled transfer of an edge detection signal with a set of transformer coils by causing a current to flow through the coils in opposite directions for the rising and falling of an input signal (see Patent Literature 1). When the current increases in the positive direction in accordance with the rising of the input signal, a positive voltage is generated in the secondary coil. When the current decreases toward zero afterwards, a negative voltage is generated in the secondary coil.
The positive voltage provides the required edge detection signal, while the negative voltage is unwanted noise. At the falling of the input signal, a negative voltage which is the edge detection signal and a positive voltage which is noise are generated in the secondary coil. To negate the noise, a non-detection period is provided for the secondary side circuit. However, when short pulses appear in the input signal, the next edge detection signal will be included in the non-detection period, because of which this edge detection signal is made invalid and a detection failure may occur.
In another configuration that has been proposed, while it adopts the method of using a current flowing through the coils in opposite directions for the rising and falling edges of the input signal, the gradient of the decreasing coil current is made smaller than the gradient of the increasing coil current (see Patent Literature 2). According to this method, the noise component generated in the secondary coil is reduced, so that the necessity of the non-detection period is eliminated.
In the configuration described in Patent Literature 2, an H bridge is used to flow a current through the primary coil. When short pulses appear in the input signal, the input signal may fall during the decrease of the coil current started to increase in response to the rising of the input signal, which may cause arm short-circuiting of the H bridge. Patent Literature 2 does not mention the timing of switching the standby switch that forms the lower arm of the H bridge. If the standby switch is turned off during the decrease of the coil current (if the switching timing is too early), an oscillatory noise may be generated on the secondary side, which may lead to generation of a wrong output signal. On the other hand, if the switching timing is too late, errors may occur when short pulses appear in the input signal.